1. Technical Field to Which the Invention Pertains
The present invention relates to an engine water pump drive structure in which a crank pulley fixed to an engine crankshaft and a water pump pulley connected to the pump shaft of the water pump supported in an engine block are connected to each other via an endless belt.
2. Description of the Related Art
A water pump for circulating cooling water to an engine water jacket is driven by connecting a water pump pulley fixed to a pump shaft and a crank pulley fixed to a crankshaft via an endless belt. As is conventional, the rotational speed of the water pump changes in proportion to the rotational speed of the engine crankshaft.
When the rotational speed of the water pump changes in proportion to the rotational speed of the engine crankshaft, the rotational speed of the water pump decreases when the engine is operated at low speed, and the rotational speed of the water pump increases when the engine is operated at high speed. If the diameter of the water pump pulley is made small so that the required rotational speed of the water pump can be guaranteed when the engine is operated at low speed, the rotational speed of the water pump becomes excessive when the engine is operated at high speed, thereby causing an energy loss. If the diameter of the water pump pulley is made large in order to prevent this, not only is it impossible to guarantee that the required amount of cooling water will be circulated when the engine is operated at low speed, but also the water pump pulley projects out to the side of the engine block thereby resulting in an increase in the overall size of the engine.
Cutting off the connection between the water pump pulley and the pump shaft by placing an ON/OFF type magnetic clutch of the type used in an air conditioner compressor between the water pump and the pump shaft could be considered, but it is difficult to precisely control the rotational speed of the water pump by means of an ON/OFF type magnetic clutch.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to maintain an appropriate rotational speed for the water pump over a wide range of engine rotational speeds without increasing the size of the water pump pulley.
In order to achieve the above-mentioned object, in accordance with the disclosed invention, there is provided an engine water pump drive structure in which a crank pulley fixed to a crankshaft of an engine and a water pump pulley connected to a pump shaft of a water pump supported in an engine block are connected to each other via an endless belt, said drive structure comprising a fluid coupling provided between the pump shaft of the water pump and the water pump pulley, said fluid coupling including means for decreasing the rate of increase in the output rotational speed of the fluid coupling as the input rotational speed increases.
In accordance with the above-mentioned arrangement, since the fluid coupling provided between the pump shaft of the water pump and the water pump pulley has the property of decreasing the rate of increase in the output rotational speed of the fluid coupling as the input rotational speed increases, the required rotational speed of the water pump can be guaranteed when the rotational speed of the crankshaft is low, and the water pump can be prevented from rotating excessively when the rotational speed of the crankshaft is high. Moreover, since the diameter of the water pump pulley can be reduced and, in addition, the water pump can be prevented from rotating excessively when the crankshaft is rotating at high speed, the amount of overhang of the water pump pulley relative to the engine block can be decreased, thereby reducing the overall size of the engine.
The invention also provides an engine water pump drive structure including means for decreasing the amount of slip of the fluid coupling as the temperature of the air surrounding the engine rises.
In accordance with the above-mentioned arrangement, since the amount of slip of the fluid coupling decreases as the temperature of the air surrounding the engine rises, it is possible to effectively prevent the engine from overheating by rotating the water pump at high speed when the temperature of the engine is high.
Still further, there is provided an engine water pump drive structure wherein the fluid coupling comprises a liquid chamber and a liquid reservoir rotating integrally with the water pump pulley, a rotor housed in the liquid chamber and rotating integrally with the pump shaft, a communicating passage for providing communication between the outer end of the liquid chamber in the radial direction and the liquid reservoir, a valve hole formed in a partition separating the liquid chamber from the liquid reservoir, a control valve for opening and closing the valve hole, fan blades rotating integrally with the water pump pulley and taking in air, and a bimetal operating in response to the temperature of the air taken in by the fan blades so as to operate the control valve.
In accordance with the above-mentioned arrangement, when the temperature of the air surrounding the engine decreases, the bimetal deforms to close the control valve, so that communication between the liquid reservoir and the liquid chamber is blocked. As a result, even when the fluid inside the liquid chamber is discharged into the liquid reservoir by the pumping action of the rotor, since the fluid is not returned to the liquid chamber, the torque transmitted to the rotor decreases, the rotational speed of the pump shaft decreases, and the warm-up of the engine can thus be accelerated effectively. When the temperature of the air surrounding the engine increases, the bimetal deforms to open the control valve, so that communication is provided between the liquid reservoir and the liquid chamber. As a result, when the fluid inside the liquid chamber is discharged into the liquid reservoir by the pumping action of the rotor, since the fluid is returned to the liquid chamber, the transmission of torque to the rotor is ensured, the rotational speed of the pump shaft is maintained, and the occurrence of overheating can be effectively prevented.